Impact from glacial rock flour on phytoplankton growth and the carbonate system

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Glacial rock flour (GRF) is a felsic, silicate sediment that originates below the Greenland Ice Sheet, where the ice abrades basement rocks to a very fine powder. This is then transported by meltwater rivers or subglacial discharge into fjords and coastal waters. Thus, GRF is a naturally occurring component of the oceans around Greenland. The grain size of GRF typically ranges <1 - 100 µm with a median of 2-5 µm. The material behaves colloidally in water and distributions in fjords and coastal waters show that it has a residence time in the surface layer of up to several weeks. Glacial rock flour deposits are voluminous and common along the coast of Greenland and therefore it has the potential to be applied in geoengineering efforts on a global scale. The potential alkalinization from conservative cation release is estimated to be ~5,000 moles of alkalinity produced per ton of dissolved GRF. Additionally, GRF contains silica and phosphate that may contribute with macronutrients for phytoplankton growth together with various trace metals, e.g., iron and manganese. Hence, adding GRF to ocean surface waters has the potential to influence phytoplankton growth and, at the same time, increase alkalinity. However, the physical and chemical cycling of GRF in the water column, its implications for ecosystem services, and the chemical impact on the carbonate system are not well understood. The first results from incubation experiments with GRF in the field and from controlled laboratory experiments are presented here. Incubation experiments of GRF added to seawater collected in the Canary Current system showed a significant increase in photosynthetic activity during short term (~1 week) incubations. The positive influence from GRF on phytoplankton biomass and photosynthetic activity is also found in incubation experiments with a monoculture of a green planktonic alga and shows that trace metals mobilized within a few weeks have a significant positive effect on phytoplankton growth. Laboratory experiments of the settling rate of GRF show that the residence time is relatively long but also that flocculation of GRF particles, caused by salinity increases, may be an important process to consider in future field studies. Our results show that GRF has significant potential for increasing alkalinity, and that trace metals are mobilized from GRF in seawater, which can stimulate photosynthesis. We propose that GRF has the potential to impact ecosystem structure and increase biological productivity when applied to the ocean.